2,5-Furandicarboxylic acid (FDCA) is a chemical in which interest has been growing recently. It is for instance seen as an alternative for terephthalic acid in the manufacture of polyesters. The advantage of FDCA resides in that it can be obtained from renewable resources, whereas terephthalic acid is obtained from p-xylene that is recovered from fossil fuels. FDCA can be synthesized by the oxidation of 5-hydroxymethylfurfural (HMF) and/or ethers and/or esters thereof. The oxidation reaction has inter alia been described in WO 2011/043661 and U.S. Pat. No. 8,519,167.
In such oxidation reactions not only FDCA is produced, but also some by-products, including colorants, also known as color bodies. Another one of such by-products is 5-formyl-2-furan carboxylic acid (FFCA). This by-product is a mono-carboxylic acid. Its presence in an FDCA composition is undesirable, since the mono-carboxylic acid will terminate the chain growth during polymerization. It has been found that it is difficult to remove FFCA by crystallization from an acid composition comprising FDCA and FFCA. This problem has been acknowledged in the art. In U.S. Pat. No. 8,969,404 a purification method is described wherein a crude composition that contains FDCA, is subjected to selective hydrogenation in order to convert by-products, such as color bodies and FFCA, to more innocuous products that can be more easily separated from FDCA. This purification method is analogous to the purification of crude terephthalic acid that contains 4-carbonyl-benzoic acid as by-product. In the case of terephthalic acid 4-carbonyl-benzoic acid is selectively hydrogenated to p-toluic acid, which can be easily separated from terephthalic acid.
U.S. Pat. No. 8,969,404 teaches that the hydrogenation of FFCA may result in 5-hydroxymethyl-2-furancarboxylic acid (HMFA), 5-methyl-2-furancarboxylic acid (MFA), 2-hydroxymethylfuran and furan-2-carboxylic acid (FCA). In addition, the hydrogenation may also lead to the saturation of the furan ring in FDCA, leading to 2,5-tetrahydrofuran-dicarboxylic acid (THFDCA). According to U.S. Pat. No. 8,969,404 all these hydrogenation products can be easily separated from FDCA through any number of techniques, such as crystallization. The hydrogenation can be carried out at a very broad range of conditions, viz. at a hydrogen partial pressure of 10 to 900 psi (0.69 to 62.05 bar), at a temperature of 130 to 225° C. for a period in the range of 15 min to 10 hrs. In experiments hydrogenation reactions are conducted at a hydrogen partial pressure of 75 to 231 psi (5.17 to 15.93 bar) and a temperature of 150 to 200° C. for 1 or 3 hrs. When higher pressures were used the results deteriorated significantly since excessive ring hydrogenation occurred to form THFDCA.
By means of these experimental data U.S. Pat. No. 8,969,404 suggests that the amount of THFDCA can be controlled at high hydrogen partial pressure or high hydrogenation temperatures by limiting the amount of hydrogenation catalyst and by limiting the residence time. Although U.S. Pat. No. 8,969,404 shows the amounts of FFCA and THFDCA in hydrogenated FDCA compositions, it fails to show what the levels of other by-products are in the hydrogenated and purified FDCA composition. Neither does it mention what the loss of FDCA is at the reaction conditions applied. It does mention though that severe hydrogenation conditions, i.e. at high temperature and in the presence of palladium catalyst, decarboxylation or hydrogenolysis of FDCA or THFDCA may occur. It is evident that such decarboxylation reaction leads to further losses of the desired FDCA product. U.S. Pat. No. 8,748,479 teaches a similar process.
US2014/128623 claims the preparation of esters of purified FDCA as plasticizer. It describes a possible method of producing purified FDCA by hydrogenation of contaminated FDCA and separating a hydrogenation species from the hydrogenation product. No indication has been given what species is separated nor how the species is separated. The specification teaches that the hydrogenation reaction takes four hours.
US 2014/142328 discloses a process to produce a dry purified carboxylic acid product comprising furan-2,5-dicarboxylic acid (FDCA) by oxidizing a feed stream comprising at least one oxidizable compound to generate a crude carboxylic acid slurry comprising FDCA. Impurities are removed therefore via oxidative purification in a post-oxidation zone. In addition, FDCA may further be decolorized via hydrogenation. No further details on the hydrogenation have been provided.
As indicated in U.S. Pat. No. 8,969,404 the crude acid composition comprising FDCA may be obtained from the catalytic oxidation of 5-hydroxymethyl furfural (HMF) or esters or ethers thereof or mixtures of HMF and an ester or ether thereof. It is known that the oxidation of ethers of HMF results in a mixture of FDCA and an ester thereof. Such is shown in e.g. U.S. Pat. No. 8,519,167. According to U.S. Pat. No. 8,969,404 the oxidation product which is a crude FDCA composition which will then also comprise an amount of FDCA ester, is combined with a hydrogenation solvent to obtain solvated FDCA and FFCA. Solvated FDCA also contains esters of FDCA. The solvated combination is then subjected to hydrogenation, which according to U.S. Pat. No. 8,969,404 may take 1 to 3 hours. It is undesirable to subject the solvated FDCA composition to catalytic hydrogenation since it has been generally described that the catalytic conversion of carboxylic acids to alcohols is more difficult than the conversion of esters to alcohols. Such is confirmed in U.S. Pat. No. 6,441,241. Hence, if the solvated FDCA composition, comprising FDCA, FFCA and esters of FDCA and, optionally, of FFCA, is subjected to catalytic hydrogenation, as taught in U.S. Pat. No. 8,969,404, there is a possibility that the esters that are present in the solvated FDCA composition will be converted to the alcohols, which leads to a yield loss. Therefore, the present inventors have found that such solvated FDCA compositions are to be subjected to a hydrolysis step before the resulting converted composition is subjected to hydrogenation.
Further, in confirmation of the teachings of U.S. Pat. No. 8,969,404, it has been found that some FCA is formed during the hydrogenation of an FDCA-containing composition. FCA, being a monocarboxylic acid, is as undesired as FFCA. It has now appeared that the purification of the hydrogenated product is detrimentally affected by the presence of an ester of FDCA. The more ester is present in the hydrogenated product, the more FCA is retained therein. It is therefore important to reduce the levels of ester of FDCA in the oxidation product before such oxidation product is subjected to hydrogenation in order to facilitate the separation of FDCA in the hydrogenated product in the desired purity.